1. Field of the Invention
This invention relates to a tape reel device for winding a tape material, such as an ink ribbon, therearound, which applies back tension to the tape material when it is rolled out, and a tape cartridge incorporating the tape reel device.
2. Prior Art
FIGS. 1 and 2 show a conventional tape reel device. As shown therein, a spool 53 is arranged between an upper casing 51, a lower casing 52, such that the spool 53 has opposite ends thereof rotatably supported by the upper casing 51 and the lower casing 52, respectively, with an ink ribbon R wound therearound. The spool 53 is in the form of a hollow cylinder, with its upper end being fitted on a hollow shaft portion 54 extending from the upper casing 51 such that it is rotatably supported thereby, and its lower end rotatably supported by a bearing ring 56 seated on an annular projection formed on the bottom of the lower casing 52. In other types of the conventional device, the bearing ring 56 is omitted. The spool 53 has its upper end face in contact with a plate spring 53 fitted on the hollow shaft portion 54 whereby the spool 53 is urged by the plate spring 57 toward the bearing ring 56, i.e. downward as viewed in FIG. 1.
The plate spring 57 is in the form of a slightly deflected annulus, and sandwiched between the lower surface of the upper casing 51 and the upper end surface of the spool 53 in a strongly stressed fashion. According to this arrangement, when the ink ribbon R is pulled to be rolled out from the spool 53, plate spring 57 acts to brake the rotation of the spool 53, and at the same time apply back tension to the ink ribbon R to thereby prevent backlash of the spool 53 and loosening of the ink ribbon R.
The conventional tape reel device as described above requires the plate spring 57 to be arranged in the narrow spacing between the upper casing 51 and the spool 53, as means for braking the spool 53, so that the plate spring necessarily has a short spring stroke (amount of deflection when the spring is urged) and a large spring constant. The necessity of obtaining a predetermined spring force in the short spring stroke results in an increased deflecting load per unit stroke (i.e. spring coefficient), which means that a slight change in the amount of deflection drastically changes the urging force of the plate spring 57. Therefore, unless the flatness of the lower surface of the upper casing 51 and the flatness of the upper end surface of the spool 53, parallelism between these surfaces, and the deflection of the plate spring 57 are attained with accuracy, the urging force of the plate spring largely changes with rotation of the spool 53, causing unstable braking force of the tape reel device.
Further, the plate spring 57 per se is in contact with the upper casing 51 at two points, and in rolling contact with the spool 53 at other two points positioned outward of the two points. This construction of the tape reel device prevents uniform urging force from being applied to the spool 53. Further, if the plate spring is formed of metal, and the spool 53 and the upper casing 51 are formed of resin, the spool 53 wears as it rotates, and if the plate spring 57 freely rotates with the spool 53, the upper casing 51 wears. This also causes unstable braking force of the tape reel device. The unstable braking force causes an extremely large back tension applied to the ink ribbon R to put large load on a drive device, or an extremely small back tension applied to the same to loosen the ink ribbon R, causing an unstable feed of the ink ribbon R.